Electrical measuring system



July 14, 1936, A. G. JENSEN 2,047,782

' ELECTRICAL'MEASURING SYSTEM K Filed May 14, 1935 s sheets-sheet 1 @WFM ATTORNEY Y' July 14, 1936 v A. G. JENSEN 2,047,782

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/NVEIVTO-R ,BV A. G. JENSEN A TTORNEV Ju1y14,1936. A, G JENSEN l 2,047,782

ELECTRICAL MEASURING SYSTEM l Filed May 14, 1955 3 Sheets-sheet z PHA sH/Fr- 25 2.3

` A A A A V V V Y V ATTEN- uATon PHASE .SH/Frm aETEc'ron 24 srs TEM l AAAI I Afro/mfr Patented July 14, 1936 zama ELECTRICAL MEASURING SYSTEMv Axel G. Jensen, South Orange, N. J., assigner to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application May 14, 1935, Serial No. 21,414 17 Claims. ((11. P19-175.3)

This invention relates to electrical measuring systems. and more particularly to such a system adapted to measure transmission characteristics at high frequencies.

An objectief the. invention is to determine the tus. A

Another object is to determine the phase ference between two alternating currents.

Another object is to increase the accuracy with which measurements of transmission loss and: phase shift may be made.

A further object is to simplify the making of such measurements.

A feature of the invention is an electrical testing circuit in which the effects of frequency drift in the signal generator are eliminated.

In electrical communication systems there is often encountered the problem of ascertaining the transmission characteristics of the component apparatus comprising the system. Heretofore difficulty has been encountered in making such measurements at high frequenciesl because of the inaccuracies resulting from the frequency drift in the signal generator. l v

In accordance with the present invention there transmission characteristics of electrical appara.-v

is provided a. method of and apparatus for mak- `makes use of two oscillators and three modulators. A portion of -the signal current from the first oscillator is fed through the apparatus `under test to the first modulator, and another portion is fed through a standard network or directly to the second modulator. The beating current for both of these modulators is obtained from the second oscillator, which has a `low frequency compared to that of the signal,

and is passed through variable attenuators and phase Shifters before it reachesv the modulators.

detected atfrequency which is a harmonic' of the second oscillator. In a modified form of the invention the network under test is connected into circuit after, instead of before, the ilrst modulator.

In the Voperation of the system, the attenuators and phase Shifters are adjusted until the detected current becomes zero. The transmission loss and phase shift introduced by the apparatus under test are then found directly from the settings of the attenuators and phase shifters. It will thus be seen that a drift in the frequency of the testing current will not aect the accuracy ofthe measurements, since only current of a comparatively low frequency passes through the variable attenuators and phase Shifters, and a harmonic of this frequency is the one which is detected. In this way a simple and accurate method is provided for measuring the transmission characteristics of apparatus at high frequencies.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings of which: i

Fig. 1 is a schematic diagram showing an embodiment of the invention in a measuring circuit in which the apparatus under test is connected into circuit-ahead of the first modulator;

Fig. 2 represents diagrammatically a modifiedl form of'the invention in which the apparatus under test is connected into circuit afterthe first, modulator;

Figs. 3 and 4 show measuring circuits in accordancel with the invention in which balanced modulators of the common input type are employed;

Figs. 5 and 6 show measuring systems using balanced modulators of the conjugate input type.; and

Figs. 7, 8 and 9 show in more detail modulators of the conjugate input type suitable for use in the measuring circuits of Figs. 5 and 6.

In the embodiment of the invention shown in Fig. 1. two oscillation generators and three modu-V lators are employed, the network il under test being connected into the circuit ahead of the rst modulator by means of the four terminals l2, i3, i4 and i5. The testing current is supplied by the generator G1, the output of whichis split into two portions, one of which passes through the apparatus Il under test to the modulatorA l5 while the other portion passes through astandard network I'I to the modulator i8. These two modulators are suppliedwith a beating current from the same generator G2 which has a lo'w frequency compared to that of the generator G1.

A variable attenuator I9 Aand a variablephase 55 shifter 28 are interposed between the generator G: and the modulator I8, and a second variable attenuator 2| and a second variable phaseshifter 22 are inserted in the circuit between Gn and the modulator I8. The output currents from the modulators I6 and I8 are combined in the modulator 23 in which all of the undesired modulation products are balanced out leaving only those which are second harmonics of the frequency supplied by the generator G2. This second harmonic is detected in the detector system 24 which may, if required, include an amplifler and a selective netwprk for suppressing all frequencies except the second harmonic. The standard network I'I introduces a known amount of attenua.- non and phase shift. Its function is to extend the range covered by the measuring system, and

in some instances it may be omitted entirely.

The method of making a measurement with the testing circuit shown in Fig. l consists in adjusting the variable attenuators and variable phase shifters until no current is detected in the detector system 24. Under these conditions the difference between the phase shift introduced by the standard and the phase shift introduced by the network under test is equal to 180 degrees plus or minus the difference between the settings of the two variable phase shifters. Likewise, the difference between the insertion loss introduced by the standard network and the insertlon loss introduced by the network under test is equal to the difference between the readings of the two variable attenuators.

'I'he tneory of operation of the measuring system shown in I" u. 1 will now be considered. The signal current fui.."hed by the generator G1 has a frequency f1 and the beating current furnished v the generator G2 has a frequency fz, which l.. small compared with the frequency f1. It is assumed that the outputs from the generators G1 and Gn'are purified of disturbing harmonics, if necessary. In the present discussion, it is also assumed that the modulators Mi and M2 are perfectly balanced, but, as pointed out hereinafter, accurate measurementsmay be obrained even though this condition is not completely fulfilled. The modulators may, for exlSimple, be of the common input type or of the onjugate input type. The output currents from the modulators I6 and I8 will principally consist of the frequencies f1, f2, (fi-l-fz) and (fi-fz) In the present arrangement, however, the frequency /1 is suppressed byproperly balancing the modulators I6 and I8 while the frequency fz is efIectively suppressed byl the selective circuit located at the input of the modulator 23 as described hereinafter. All higher order modulation products are either balanced out or kept negligibly low by proper operation of the modulators. The outputs of the modulators I6 and I8 are combined at the input of the modulator 23 and the output of this modulator 23 will contain, among other low frequency components, current of the frequency 2f2. By analyzing the two inputs to the modulator 23 each consisting of the three frequencies fr, (fi-'.fz) and (fr-l-fz), it is found that the output of frequency 2f: is balanced out when the relative .amplitude and phase shift of the two inputs are adjusted so as to make either (fi-f2) or (fi-H2) from the modulator I6 equal in magnitude and opposite in phase to the same frequency output of the modulator I8. It is thus found that there are two conditions under which a balance may be obtained. 4

The output of frequency 2f: is entirely independent of any slight drift in the frequency of the signal generator Gi since it is obtained simply as a second harmonic of the low frequency beat generator G1. Another advantage of the circuit is that the variable phase shifters and 5 variable attenuators are inserted directly in the output of the beating generator G: and thus are not subjected to any frequency change at all since this generator is operated at a frequency sufficiently low to insure stability of operation. 10

In some cases it may be found of advantage to insert the network II under test and the standard network I1 after the rst stage of modulation as is shown in Fig. 2. 'I'his method gives the same results as the one described above in 15 connection with Fig. 1 provided that the network under test oifers the same phase shift and attenuation to both of the sidebands (fi-H2) and (f1-f2). Since the frequency f2 is chosen to be low compared to the frequency fr this will be 20 substantially true in most cases.

Fig. 3 shows a measuring circuit in accordance with Fig. 1 in which the modulators I6 and I8 are indicated diagrammatically as of the cornmon input type and are of balanced construction. For the sake of simplicity in Fig. 3, as well as in the other figures which follow, the sources of electromotive force are omitted. The modulators I6, I8 and 23 are of standard construction and employ voltages of the magnitude usually used in modulators of this type. In the system shown in Fig. 3 the network under test is inserted in the circuit ahead of the modulator I6 and the system is particularly suitable for making measurements on a network of the bal- 35 anced type. At the input to modulator 23 is located a transformer 25 having a shield 26 between the primary and secondary windings connected to the grounded side of the primary. The shield may be grounded as shown at 21, either directly or through a by-.pass condenser. The secondary of the transformer 25 is tuned by means of the condenser 28 to the .frequency f1 while effectively suppressing the frequency f: and all other low frequencies.

Fig. 4 shows how the system of Fig. 3 may be modied when it is desired to make measurements on networks of the unbalanced type. In Fig. 4 the network I I under test and the standard network I'I are located respectively after the modulators I8 and I8. 'I'he grounded side of the network under test may be connected to ground as shown at 5I. In other respects the measuring circuit'shown in Fig. 4 is the same as the one described above in connection with 55 F18. 3.

Fig. 5 shows a modication of the invention in accordance with Fig. 2 using modulators of the conjugate input type in which the signal frequency f1 is impressed upon the midbranch. The 60 system shown in Fig. 5 is particularly suitable for making measurements on balanced networks. In this circuit the primary of the transformer 25 is grounded at the midpoint as shown at 29, either directly or through a by-pass condenser. 65 In other respects the circuit is similar to those described above in connection with Figs. 3 and 4.

Fig. 6 shows how the measuring circuit of Fig.

5 may be modified when it is desired to make measurements on unbalanced networks. 'I'hc 70 network under test II and the standard network II are connected into the circuit between the signal generator Gx and modulators I6 and I8 respectively. The grounded side of the network under test may be connected to ground 75 aosivea 'as simwn at u. In this usure the midpoint of` virom the generator Gi is impressed upon the modulator by means of the pair of transformers 31 and 38, the primary windings of which are connected in parallel as shown. Current from the beating oscillator Gn is impressed upon the modulator by means of the transformer 39 the secondary of which is divided and tuned by means of the condensers 40 and 4I.

Other circuit arrangements suitable for lthe modulators I6 and I8 of Figs. 5 and 6 are shown in Figs. 8 and 9, in which pentodes are used. In Fig. 8 two space charge pentodes 42 and 43 are employed with thelow frequency applied through the balanced transformer 44 to the space charge grids 45. Fig. 9 is similar to Fig. 8 except that two power pentodes 4B and 4'I are used, the low frequency being applied to the suppressor grids 48. In Figs. 8 and`9 the reference numeral 49 indicates the control grid, and 50 the screen grid.

Even though the modulators I6 and I8 have slight inequalities andthe two halves of the measuring circuit have slight amplitude and phase imbalances, a correct result may be obtained by taking the average of two measurements, one oi which is made after interchanging the leads from the networks II and I1 tothe modulators I6 and I8. Another way of obtaining the correct answer is to make a separate measurement with the unknown network and the standard network removed and to apply this zero correction to the measurement obtained with the networks connected into the circuit.

What is claimed is:

l. A method of determining the phase difference between two alternating currents which consists in separately combining said two currents with separate portions of a. third current to produce resultant waves, combining'said resultant waves to produce a harmonic of said third current, and adjusting the phase diierence between the separate portions of saidA third current until said harmonic disappears.

2. A method of determining the phase dierence between two alternating currents which consists in separately combining said two currents with separate portions of a. third current to produce resultant waves, combining said resultant waves to produce the second harmonic of said third current, and adjusting the phase difference between the separate portions oi' said third current until said harmonic disappears.

3. 'I'he method of determining the phase din'erence between two Avibrations of the same frequency which consists in producing beats between each of said vibrations and separate portions of a third vibration, combining said beats to produce a vibration which is a harmonic `oi' said third vibration, and adjusting the phase difference between the separate portions of said third vibration until said harmonic disappears.

4. 'I'he method of determining the phase difference between two alternating currents of the same frequency which consistsin heterodyning each current with a third current of low frequency compared to said mst-mentioned curbeats to produce a harmonic is eliminated.

rents. combining the trains of beat frequencies thus produced, detecting a harmonic frequency of said third current, and adjusting the phase of said third current until said harmonic frequency disappears. 5 5. The method of determining the phase and amplitude differences between two alternating currents of the same frequency which consists in heterodyning each of said currents with separate portions of a third current the phase diiferl0 ence and relative amplitudes of which may be' adjusted, combining the two resultant trains of harmonic frequency .of said third current, and adjusting the phase ditlerence and relative amplitudes of the separate portions 15 of said third current until said harmonic frequency is balanced out.

6. A method of determining the phase and amplitude diiferences between two alternating currents which consists in separately combining 20 said two currents with separate portions Iof a third current to produce resultant waves. combining said resultant waves to-producea harmonic of said third current, and adjusting thel phase difference and relative amplitudes of the 25 separate portions of said third current untilsaid harmonic is eliminated.

7. A method of measuring the phase shift in electrical apparatus which consists in generating a. testing current, dividing said current into two 30 portions, sending one of said portions through said apparatus, separately modulating said two portions `oi.' current with separate portions of a third current, combining the resultant waves to produce a harmonic of said third current, and 35 adjusting the phase difference between the separate portions oi' said third current until said harmonic disappears.

8. A method of measuring t-he phase shift and insertion loss introduced by electrical apparatus which consists in generating a testing current, dividing said current into two portions, separately modulating said two portions of current with separate portions of a beating current to produce resultant trains of beat, passing one of said trains of beats through said apparatus, combining said resultant trains of beats to produce a harmonic of said beating current, and adjusting the phase difference and relative amplitudes of the separate portions of said beating current until said harmonic is eliminated.

9. .d method of measuring the phase shift and insertion loss introduced by electrical apparatus which consists in generating a testing current, dividing said current into two portions, passing one of said portions through said apparatus, sepa.- rately modulating said two portions of current with separate 1 portions of a beating current to produce resultant trains of beats, combining said resultant trains of beats to produce a harmonic 60 of said beating current, and adjusting the phase difference and relative amplitudes of the separate portions of said beating current until said 65 10. A system for measuring the phase diiTerence between two alternating currents comprising two modulators, means for supplying said two currents to each of said modulators, a source of beating current, means for supplying separate 70 portions of said beating current to each of said modulators, means for varying the phase difference between the separate portions of said' beating current, means for combining the outputs of said two modulators to produce a har- `75 monic of said beating current, and means for detecting said harmonic.

11. A system for measuring the phase difference between two alternating currents comprising two modulators. means for applying said two currents to each of said modulators, a source of beating current, means for supplying separate portions of said beating current to each of said modulators, means for varying the phase difference between the separate portions of said beating current, means for combining the outputs of said two modulators to produce the second harmonic of said beating current, and means for. detecting said second harmonic.

12. A system for determining the phase and amplitude diierence between two alternating currents comprising a. source of low frequency current, means for separately combining said low frequency current with each of said mst-mentioned currents to produce beats, means for combining said beats to produce a harmonic of said low frequency current, and means for varying the phase and amplitude of said low frequency current.

13.' A system for determining the phase and amplitude differences between two alternating currents of the same frequency comprising two modulators, means for supplying said two currents separately to said modulators. a source of heterodyne waves, means for varying the phase and amplitude of said heterodyne waves. means for supplying said heterodyne waves to said modulators, a third modulator, means for supplying the output currents from said two modulators to said third modulator, and means for detecting a harmonic vfrequency of said'heterodyne waves.

14. A system for determining the phase shift in electrical apparatus comprising a source of testingcurrent,meansfor sending a portion of said current through said apparatus under test, a source of beating current, means for combining a portion of said beating current with said portion of testing current to produce a train of beats, means for separately combining a second portion of said beating current with another portion of said testing current to produce a second train of beats, means for combining said two trains of beats to produce a harmonic of said beating current, and means for adjusting the phase dierence between the two portions of said beating current.

15. A system for measuring the phase shift and attenuation of electrical apparatus comprising a solu'ce of testing current, two modulators, means for sending a portion of said testing current through said apparatus to one of said modulators. means for supplying another portion of said test- 5 ing current to said second modulator, a source of beating current, means for supplying separate i portions of said beating current to each of said modulators, means for combining the output currents from said two modulators to produce a harmonic of said beating current, and means for varying the phase diiference and relative amplitudes of the separate portions of said beating current.

16. A system for measuring the phase shift and attenuation of electrical apparatus comprising a source of testing current, two modulators, means for supplying a portion of said testing current to one of said modulators, means for supplying a second portion of said testing current to said second modulator, a source of beating current, means for supplying separate portions of said beating current to each of said modulators, means for sending the output current from one of said modulators through said apparatus, means for combining the output currents from said two modulators to produce a harmonic of said beating current, and means for varying the phase difference and relative amplitudes of the separate portions of said beating current.

17. An electrical measuring system for determining the phase shift and insertion loss introduced by electrical apparatus comprising an oscillator, a second oscillator of low frequency compared 'to said first-mentioned oscillator, means 85 for combining a portion of the output of said first oscillator with a portion of the output of said second oscillator to produce a train of beats, separate means for combining another portion of the output of said first oscillator with another por- 4( tion of the output of said second oscillator to produce a second train of beats, a modulator, means for passing one of said trains of beats through said apparatus to the input of said modulator, means for supplying said second train of beats to the input of said modulator, means for detecting the output of said modulator, and means for varying the phase and amplitude of at least one of said portions of the output of said second oscillator.

AEL G. JENSEN. 

